1. Field of the Invention
The present invention relates to a fixing apparatus, which is equipped in, for example, an electronic photocopier.
2. Description of the Related Art
Some fixing apparatus of the above-mentioned type include a metallic heat roller and an elastic pressure roller that is set in pressure contact with the heat roller. The heat roller comprises a halogen lamp or the like, serving as a heat source, and it is heated by the radiant heat of the halogen lamp.
Paper sheets on which toner images have been transformed are allowed to pass between the heat roller and pressure roller, and thus each sheet is heated and pressurized, thereby fixing the toner image on the sheet.
On the other hand, according to conventional techniques, the heat roller is heated by heating the air within the heat roller by means of light radiated from the halogen lamp. With this structure, when the light is converted into heat energy, heat loss occurs. Further, the efficiency of transmitting heat to the heat roller is poor. Consequently, the heat exchange efficiency is as low as 60 to 70%, and thus the conventional techniques have a disadvantage in terms of energy conservation.
Further, because of the low heat exchange efficiency, the conventional techniques entail a drawback of requiring a great amount of time to warm up the fixing apparatus.
Under the circumstances, recently, a fixing apparatus that utilizes the induction heating method has been developed as discussed in Jap. Pat. Appln. KOKAI Publication No. 9-258586.
Jap. Pat. Appln. KOKAI Publication No. 9-258586 discloses a fixing apparatus that comprises a coil assembly obtained by winding a coil around a core member, which is provided along the rotation shaft of the fixing roller. Here, the fixing roller is heated with the coil assembly by allowing an eddy current to flow to the assembly.
In such a prior art technique, with a structure of winding a coil around a core member, the magnetic flux generated can be concentrated and thus a high output can be obtained. However, at the same, due to the structure in which a core member with a coil wound therearound is placed in the device in the prior art technique, it would require, in addition to the heavy core member itself, a bobbin for winding the coil and a holder for holding the core member. As a result, the heat roller is increased in size as well as in weight.
There has been an attempt of thinning the core member in order to reduce the size and weight of the heat roller. However, in this case, it brings another drawback of creating magnetic saturation since the temperature of the core member is easily increased during the process of heating the heat roller. Consequently, the output range of the excitation coil is lowered, thereby making not possible to heat the heat roller sufficiently.